In a so-called character display apparatus for displaying a very small pattern such as character, symbol, figure and the like in a terminal display apparatus, in order to afford a reproduced picture image of high definition, a 2 field 1-frame interlaced scanning system is not used but a scanning system which sequentially scans the scanning lines is employed to avoid the scanning lines of each field being displaced from each other in position. In order to obtain a display screen which provides a frame number same as that of the prior art interlaced scanning system, the scanning speed of the electron beam must be twice as high as that of the prior art. This causes the signal band width to become large or causes the power required for the deflecting means to become large. Therefore, in the character display apparatus serving as the terminal display apparatus of this kind, the still picture is used very frequently so that the display screen is reenergized about 30 to 50 times per second. Accordingly, if the display apparatus of this kind uses a phosphor that is generally used in the prior art television picture tube of an interlaced scanning system, the picture image will flicker considerably. Therefore, a cathode ray tube of the apparatus of this kind uses as its phosphor a long-persistence phosphor whose decay time lies in a range from several milli seconds to several tens of milli seconds. However, if the address detection is carried out by a light pen as one of the using modes of use of the character display apparatus, since it is necessary that the moment when the electron beam passes through the phosphor screen must be detected by the light pen, in the range where an output is produced, the decay time of the phosphor must be as short as possible so as to increase the pick-up accuracy. Thus, a short-persistence phosphor which has a decay time on the order of a micro (.mu.) second is necessary.
Generally, the decay time represents the time until the brightness at the initial light emission is decreased to 1/10 thereof.
As described above, the cathode ray tube of this kind requires a long-persistence phosphor to make a picture image which can be seen by naked eyes and a short-persistence phosphor to carry out the address detection by using a light pen because a detection pulse of very short time must be provided. Therefore, these requirements are conflicting with each other.
Accordingly, in the cathode ray tube of this kind, as the green and red phosphors which form its phosphor screen, for example, color phosphor screen, long-persistence phosphors are used while as the blue phosphor having relatively low visibility, a short-persistence phosphor is used. In this case, however, notwithstanding the low visibility of the blue phosphor, it is a short-persistence phosphor so that there is caused a slight flicker in the picture image, too.
In another prior art example, a long-persistence phosphor and a short-persistence phosphor of the color same as that of the long-persistence phosphor are mixed and used, in which the detecting means detects only the level at which both phosphors are rendered luminous at the same time. In this case, however, it is practically very difficult to select the mixture ratio of both of them such that the flicker of the picture image can be suppressed, while the pick-up accuracy can be increased. If the mixed amount of short-persistence phosphor is very large, the flicker of the picture image becomes conspicuous. If, on the other hand, it is small, the S/N grows worse to lower the pick-up accuracy of the light pen whereby the positive address detection will not be made without difficulty.
To overcome such shortcomings, it may be considered that a phosphor screen is formed by separately coating on the inner surface of the panel of a cathode ray tube phosphors with decay time of, for example, longer than 1 milli (m) second and to emit visible light, for example, light rays of red, green and blue colors by the impingement of an electron beam and a phosphor with decay time shorter than 10 micro (.mu.) seconds and to emit a detecting light, for example, ultraviolet rays, infrared rays and so on by the impingement of electron beam for the light pen pick-up outside the area of visible light. More particularly, for example, the color phosphor screen, the phosphors which are respectively rendered luminous in red, green and blue are respectively formed to have a predetermined pattern of stripe-shape or dot-shape and the phosphor which emits the afore-mentioned light pen pick-up detecting light is formed such that it exists between the adjacent color phosphors as seen from the front of the phosphor panel of the cathode ray tube.
The present invention relates to a method for producing a phosphor screen of a cathode ray tube suitable in obtaining a phosphor screen in which in the space between the phosphors with the predetermined pattern of, for example, stripe-shape or dot-shape to emit the visible light, there is separately coated a phosphor to produce the light pen pick-up detecting light like the phosphor screen of the afore-mentioned light pen pick-up system cathode ray tube. Particularly this invention is to provide a method for producing a phosphor screen of a cathode ray tube by which its phosphor pattern, particularly the phosphor pattern for the light pen pick-up use can be produced positively.